I am trying to figure out why we need URIs for XML namespaces and I cannot find a purpose for that. Can anyone brighten me a little showing their use on a concrete example?


Ok so for instance: I have this from w3schools



   <f:name>African Coffee Table</f:name>


So what should http://www.w3schools.com/furniture hold ?


4 Answers 4


A namespace is a way of saying "This kind of Foo" is different from "That kind of Foo", even though they are spelled the same. Or, if you prefer "MY kind of Foo" is different from "Everybody else's kind of Foo".

The technical way of saying this is "The URI of my namespace for Foo" is different from everybody else's URI for their namespace for Foo. In other words, URIs are just strings that allow you to say so.

The trick is then to say, "Hey, URLs are valid URIs", and then use a URI corresponding to a URL under your control. If everybody does that, then you can avoid accidental namespace collisions. You could as well have said namespace "A" and namespace "B", but you risk that somebody else would use the same namespace too, and then your kind of Foo is not different from their Foo anymore which is exactly what you want to avoid.

You can then add additional conventions to the URLs used as URIs, for instance, that the URL must correspond to a page containing documentation or XSDs or similar, but this is not necessary. It is just convenient.

  • Ok, but what's a difference between namespace A and namespace w3.org/1999/xhtml ? both of them can be used by users without them knowing that they have used the same namespaces.
    – Patryk
    Nov 28, 2011 at 4:14
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    @Patryk, that is why you use a URL you have control over. The xhtml namespace URL is under control of W3C who defined the XHTML specification.
    – user1249
    Nov 28, 2011 at 4:24
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    @Patryk, also note that it is the ones defining the namespace that gets to pick the URL. Then everybody else can use them.
    – user1249
    Nov 28, 2011 at 5:07
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    So namespace a would totally be valid? Stupid, but valid? Or does it have to be a URI?
    – Rudie
    Sep 1, 2014 at 12:29
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    Wouldn't it have been awesome if the first few people to use that convention years ago had also put nicely concise documentation of their schemas at those URLs so that became a thing? Microsoft's OpenXML namespaces schemas.openxmlformats.org/drawingml/2006/chart points to nothing.
    – prototype
    Sep 23, 2015 at 2:46

In general terms, any unique identifier would serve for a namespace; but since it's supposed to be globally unique, the standard would have to either mandate some arbitration authority, or use another resource that is at the same time globally unique but easy to get hold and to prove it's yours.

Oh, look! if you have a domain, it's obviously only yours, and it's easy to prove it's yours!

For example, they wanted to avoid the situation where two different bookstores start using "books" as a namespace, with totally different internal definitions. While they use it internally, there's no problem; but as soon as one of them wants to publish their specification, all hell breaks loose.

Instead, if you're encouraged to use your domain, anybody (well, anybody who owns a domain) can define their own private namespace with the confidence that nobody else will use the exact same identifier.

As a bonus, you can optionally store the definition of your namespace at the URI, making it self-documenting.

Seems like a great idea to me.

  • Thanks for the answer but I would need something more concrete to understand this. I added some exemplar code that you can base on, please take a look.
    – Patryk
    Nov 28, 2011 at 3:51
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    @patryk - nothing, it's not an address - it's just a unique token that you are sure no one else that you might mix xml files with has. All it is saying is that the names <table> etc won't be used wit any different meanings in any xml files that have the www.w3.org name at the top Nov 28, 2011 at 3:59
  • Ok but when wikipedia says something that users can use for instance w3.org/1999/xhtml as a namespace then a lot of people would have the same namespace. Am I right ?
    – Patryk
    Nov 28, 2011 at 4:12
  • @Patryk, only if you want to use it for the same use. If you want to define a new namespace, you're encouraged to use your own domain
    – Javier
    Nov 28, 2011 at 4:16
  • @Javier What do you mean by 'the same use' ?
    – Patryk
    Nov 28, 2011 at 4:39

DOM Level 2 was published in late 2000. It introduced the "getElementById" function as well as an event model and support for XML namespaces and CSS.

What xmlIns specifies?
As attribute nodes named "xmlns" or "xmlns:xxx", exactly as the namespaces are written in the source XML document. This is the model presented by DOM.

What URI contains?
An XML namespace is declared using the reserved XML pseudo-attribute xmlns or xmlns:prefix, the value of which must be a valid namespace name.

For example, the following declaration maps the "xhtml:" prefix to the XHTML namespace:


AS i have discovered after reading the article on Wikipedia. URI contains the XML formatted specifications of the namespace provide a method to avoid element name conflicts.

May be you know xml document uses XML Schema for the element/table/class structure of elements of the XML document.

what should http://www.w3schools.com/furniture hold ?

The answer is on your page where you have learned about the XML Namespace. Check Namespaces in Real Use section on this W3Schools article.

Check following link of stackoverflow which some what demonstrate that what this URI carries:

How to create and use XML namespace?


URI = namespace's globally unique name or identifier

Consider building a database or document on the financial management of the logistics of the oil industry. That would need to use specific vocabularies (i.e., controlled terms) for each of the subjects: database, document, management, finance, logistics, oil, and industry.

If none of the terms overlapped across these seven vocabularies, then only one URI is needed. But for each term that overlaps other terms across these vocabularies, you'd need an additional subject-specific vocabulary identified in a URI or IRI. One way around this is to create a merged taxonomy, with broader general terms and narrower subject-specific terms.

This could then be expanded into a thesaurus that also shows the jargon-synonyms of a term, and enables specification of a preferred term among those jargon-synonyms.

  • 2
    this post is rather hard to read (wall of text). Would you mind editing it into a better shape?
    – gnat
    Apr 9, 2016 at 17:42

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